Bladder cancer (BC) is a heterogeneous disease entity exhibiting divergent phenotype, biological behavior and clinical outcome. About 70% of BC present as low-grade, superficial papillary tumors that are frequently recurrent but infrequently progress to high-grade muscle-invasive stages. The rest (~30%) of BC are high-grade and invasive at presentation, and they usually do have a prior history of low-grade, papillary tumors but are believed to arise de novo or derive from flat, high-grade carcinoma in situ lesions. Whether these two major phenotypic variants of BC are caused by distinct molecular alterations is of central importance in understanding and effectively managing BC. However, answer to this question remains elusive due to the lack of concerted research efforts. Over the past 15 years, we have been dissecting the molecular mechanisms of BC pathways by developing and analyzing genetically engineered mice via urothelium-specific gene activation or ablation or both. The resulting mouse models not only recapitulate many salient features of human BC, but yield new information regarding the cause-effect relationship between genes and pathways. The present proposal is designed to gain much deeper and broader insights into the combinatorial molecular events that together serve as the drivers to trigger BC formation along the two major phenotypic pathways. Aim 1 will define the molecular alterations that are capable of synergizing with mutated fibroblast growth factor receptor 3 (FGFR3) to initiate low-grade, superficial papillary BC. This Aim will focus on (i) the deficiency of pi 51 NK4b, p16lNK4a and p19ARF, three tumor suppressors located on 9p21 and frequently deleted in early-stage human BC; and (ii) the increased signaling of mutated FGFR3 through FGF signaling. Aim 2 will examine the effects of gain-of-function p53 mutations and those in collaboration with pRb family deficiency in triggering high-grade invasive BC. A combination of cell culture, genetically engineered models and human studies will be used to accomplish the objectives. Collectively, these studies should help define the molecular drivers that are responsible for urothelial tumorigenesis along divergent pathways and help devise new biomarker panels for improved diagnosis, prognosis prediction and novel therapies of BC.